In cellular radiotelephone systems, handoff of a mobile is required to maintain communication to the mobile as it moves from cell to cell in the system. Handoff is the process of transferring a call in progress from one RF coverage area to another in coordination with the movement of the mobile. It is also the process whereby a call is transferred to another channel within a RF coverage area because of interference within the coverage area. The process of handing off a call in progress is one of the most delicately balanced function related to cellular radiotelephone systems because it requires a high level of coordination among the various system processing elements to ensure successful operation. Failure to hand a call off at the proper time generally results in a reduction in the call quality, interference with neighboring coverage areas and even the undesired termination of the call.
Current analog cellular radiotelephone systems require that the system continuously monitor the quality of every call which is operational on the system. The system must recognize when the quality of a call falls below a predetermined threshold in a particular coverage area and must also determine what other coverage area can satisfactorily handle the call. Once a more suitable coverage area is identified, the system sends instructions to the mobile directing it to another channel. The mobile confirms that it is leaving its current channel, tunes to the new channel, synchronizes to the new channel and begins transmitting thereby confirming that it has arrived on the new channel.
In digital cellular radiotelephone systems, the procedure is modified somewhat in that the mobile is capable of measuring other channels as instructed by the system as well as its current channel and also that the mobile reports this information back to the system. These measurements consist of signal strength only and are relative as the measured results are likely to vary considerably due to varying environmental conditions. Because of this and because the mobile cannot determine if the measurement is that of an interferer or the correct channel, the system must scan the cell selected by the mobile.
Another factor to consider in the handoff process is in current cellular radiotelephone systems, the radiotelephone switch receives a handoff request from the source base-site and relays the message to potential target base-sites. The target base-sites will monitor the quality of transmission of the mobile and if the required conditions are met, the target base-site will respond back to the radiotelephone switch to let it know it is a good candidate. During this time, the radiotelephone switch waits a predetermined period of time to receive additional responses from all other potential target base-sites. If no responses are received from any target base-sites, the radiotelephone switch will relay a message to the source base-site to increase the signal level of the mobile unit. The source base-site knows that a given signal level step translates to a known signal level increase at the mobile thus the source base-site sets the signal level step accordingly. The source base-site again takes signal quality measurements on the transmission of the mobile and the entire process is repeated. This process of constantly requesting a handoff when the handoff condition has been met tends to overload the radiotelephone switch since handoff information is continuously being transferred throughout the system.
Still other systems have base-sites sending all received signal strength measurements back to the radiotelephone switch on a continuous basis to speed the handoff process. This method, however, creates a large amount of signal strength data traffic which the switch must preserve.
Thus, a need exists for a handoff procedure which increases handoff efficiency by reducing the number of handoff request processed from source base-site to base-site interface while at the same time reducing the length of the overall handoff process.